Assessing humification and organic C compounds by laser-induced fluorescence and FTIR spectroscopies under conventional and no-till management in Brazilian Oxisols

Abstract

Data on humification is important to assessing the rate and magnitude of soil carbon (C) sequestration. Thus, this study assessed the humification degree (HLIF) of soil organic matter (SOM) and the changes in functional C groups (aromatic-C and aliphatic-C) for contrasting land use and management practices (native vegetation (NV), conventional plow-based tillage (CT) and no-till (NT) systems) in sub-tropical and tropical Brazilian environments. Experiments were conducted at Ponta Grossa (PG) in Paraná State and Lucas do Rio Verde (LRV) in Mato Grosso State of Brazil. Laser-induced fluorescence (LIFS) and Fourier-transform infrared (FTIR) spectroscopies, were used on whole soil samples to 1-m depth, and on seven aggregate size classes (8–19, 4–8, 2–4, 1–2, 0.5–1, 0.25–0.5, 0.053–0.25mm) obtained by wet sieving of 0–5 and 5–10cm layers. Three functional C groups were selected based on FTIR: aliphatic-C1 (1404cm−1), aromatic-C (1632cm−1), and aliphatic-C2 (2852 and 2922cm−1). The HLIF was 3 to 5 times higher at the LRV site than at PG at all soil depths, indicating that selective preservation by aromaticity of SOM is the predominant mechanism in this environment. Relatively lower HFIL was observed in NT soils at both locations because of aggregation which protects most labile moieties. The depletion of C concentration in CT was related to the decrease in functional C groups (i.e., aromatic-C and aliphatic-C) and an overall increase in the humification degree, indicating that physical protection mechanisms are not sufficient to protect the labile fractions of OM. In contrast, the intensity of functional C groups under NT systems was similar to that in the soil under NV at both locations. A discriminant analysis of principal components clearly showed that soils at both locations can be clustered into three groups, corresponding to the three main land-use and management practices. Thus, soils under NV, NT, and CT differed significantly in terms of the composition of organic compounds, and in the interactions between inorganic and organic fractions. Land use changes modify the arrangement of organic compounds necessitating the diversification of agroecosystems and conversion to NT farming. Altogether, our results reveal that LIFS and FTIR are fast, efficient, and precise techniques for analyzing the degree of SOC humification, functional C groups, and hence the efficiency of NT cropping systems in promoting long-term carbon sequestration in soils.